Substances isolated from fermented media have been found which inhibit the binding of vitamin B12 by tanscobalamins and related bitamin B12 binding proteins. This inhibition can be reversed by increasing the amount of vitamin B12 in the reaction mixture, and we conclude that these are "true" vitamin B12 antagonists. These substances will inhibit the growth of bacteria and mammalian cells in tissue culture and this inhibition is reversed by addition of vitamin B12. Since we have included the mouse leukemia cells L-1210 and P-388 in our test systems and find inhibition (reversed by B12) we may have a therapeutic approach to the control of growth of tumor cells.